Attempts to run test for python-fuelclient using tox -e cover fail with error /bin/sh: 1: coverage: not found

[Tatyanka]

tox.ini contains next env:
[testenv:cover]
commands =
    bash -c "TESTS_DIR={posargs:./fuelclient/tests/unit} python setup.py testr --coverage"

But attempts to run tox -e cover fail with:
cover runtests: PYTHONHASHSEED='2364950266'
cover runtests: commands[0] | bash -c TESTS_DIR=./fuelclient/tests/unit python setup.py testr --coverage
running testr
running=OS_STDOUT_CAPTURE=1 OS_STDERR_CAPTURE=1 OS_LOG_CAPTURE=1 ${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./fuelclient/tests/unit}
/bin/sh: 1: coverage: not found
======================================================================
FAIL: process-returncode
tags: worker-0
----------------------------------------------------------------------
returncode 127
Ran 1 (-326) tests
FAILED (id=4, failures=1 (+1))
error: testr failed (1)
ERROR: InvocationError: '/bin/bash -c TESTS_DIR=./fuelclient/tests/unit python setup.py testr --coverage'
___________________________________________________ summary ____________________________________________________
ERROR: cover: commands failed

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/253453

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/253453
Committed: https://git.openstack.org/cgit/openstack/python-fuelclient/commit/?id=d3cab164fc2bcfd92647a2a08a08771c3356fd75
Submitter: Jenkins
Branch: master

commit d3cab164fc2bcfd92647a2a08a08771c3356fd75
Author: Tatyana Leontovich <email address hidden>
Date: Fri Dec 4 13:33:54 2015 +0200

    Add coverage to test-requirement.txt

    Closes-Bug: #1522377
    Change-Id: Iaf184585a9490fbc20fead77482f34bb4c865d3c